Mesophotic mushroom coral records at Brunei Darussalam support westward extension of the Coral Triangle to the South China Sea waters of Northwest Borneo

This communication reports the discovery of two additional fungiid coral species, Cycloseris hexagonalis and Lithophyllon spinifer, from a relatively deep shelf reef in Brunei waters. These new records plus two earlier excluded ones, Cycloseris explanulata and C. wellsi, raise the known number of mushroom coral species at this Northwest Borneo location to 37 which is comparable to the number (n=35) reported for reefs of the nearby Tungku Abdul Rahman Park/Kota Kinabalu area, outside but close to the currently recognized northwest boundary of the zone of maximum marine biodiversity, the Coral Triangle (CT). The fungiid species richness at Brunei is compared with those of other sites in this eastern part of the South China Sea (SCS), and the adjacent westernmost CT ecoregion. A relatively high or comparable mushroom coral richness at all these sites, a richness which is higher than several central CT reef ecoregions, supports the argument for a westward shift of the CT boundary to the SCS waters of Northwest Borneo.